The invention relates to a sensor which includes a reference electrode as well as a plurality of sensor elements, each sensor element including a collector electrode and a photoconductor structure which is arranged between the reference electrode and the collector electrodes, as well as a switching element which connects the collector electrode to a read-out lead, the sensor elements being arranged in a matrix of n rows and m columns, where n, m are larger than 1. The invention also relates to an X-ray examination apparatus which includes an X-ray source for emitting an X-ray beam for irradiating an object so as to form an X-ray image, as well as a detector for producing an electrical image signal from said X-ray image.
A sensor of this kind is known from WO 97/10616. The known X-ray image sensor includes a matrix with a plurality of sensor elements which are sensitive to X-rays and are arranged in rows and columns. Each sensor element is composed of a collector electrode, a single-line photoconductor structure and a common reference electrode. The photoconductor layer, consisting of selenium (Se) or lead oxide (PbO), absorbs incident X-rays and generates electron hole pairs. Under the influence of a static electrical field, applied across the photoconductor layer between the reference electrode and the collector electrodes, the holes travel to the reference electrode and the electrons are collected by the collector electrodes or vice versa, depending on the relevant polarization of the electrodes. Each collector electrode is connected to a switching element. In order to read out the collected charges, the switching elements are opened and the collected charge is conducted to appropriate read-out leads, after which they flow into appropriate amplifiers which subsequently apply a charge signal to a multiplexer which converts the charge signals from the corresponding read-out leads into an electronic signal.
The sensor elements which are situated at the edge of such a sensor matrix do not always operate correctly. On the one hand, the photoconductor structure is in contact with the atmosphere, because there are no neighboring sensor elements on one side, thus enabling moisture and oxygen to exert a negative effect on the layer materials such as selenium or PbO. On the other hand, undesirable signal contributions arise because of capacitive coupling into the read-out leads and address leads connected to the collector electrodes if the photoconductor structure and/or the reference electrode extend beyond the active imaging surface. This produces undesirable noise.
Therefore, it is an object of the present invention to achieve correct operation also for the sensor elements at the edge of the matrix, irrespective of the construction of the edge of the photoconductor structure and the reference electrode.
This object is achieved by means of a sensor according to the invention which is characterized in that the matrix is provided with a guard electrode window which is arranged between the read-out lead and the photoconductor structure of the sensor elements constituting the edge of the matrix and extends beyond the edge of the photoconductor structure, i.e. beyond the edge of the active imaging surface.
The guard electrode window in a first embodiment is preferably composed of one or more large-area guard electrodes. The solution involving a large-area guard electrode offers advantages in respect of technical implementation. The overall window may be composed of a plurality of sub-windows, for example two sub-windows.
The guard electrode window in a second embodiment consists of so many individual guard electrodes that each sensor element constituting the edge of the matrix is provided with a guard electrode.
Because of the guard electrode window proposed according to the invention, disturbances in the form of parasitic capacitances between the reference electrode and the read-out leads are suppressed in the sense of shielding in the case of a non-ideal construction of the edge layer of the photoconductor structure and the reference electrode.
Moreover, the photoconductor structure may be provided so as to extend even beyond the active imaging surface, for example by the stepped stacking of several layers. This offers the advantage that material inhomogeneities at the edge of the individual layers of the photoconductor structure which are due to the manufacturing process will be situated outside the active imaging surface and will not lead to image defects. Furthermore, in a technological sense it is easier to realize a stepped configuration of a multilayer photoconductor structure than an ideal vertical section.
Preferably, the guard electrode is arranged at a layer level corresponding to the collector electrode. This enables the guard electrode window and the collector electrode to be provided simultaneously during the same process step of the manufacturing process utilizing the thin-film technique.
Furthermore, the object is also achieved by means of a sensor which is characterized in that the edge zone of the photoconductor structure of the sensor elements constituting the edge of the matrix is provided with a passivation layer which is impermeable to moisture and air, and that the photoconductor structure is composed of at least two photoconductor layers which are arranged one above the other and consist of a different material, the material chosen for one layer being lead oxide whereas the material chosen for a second layer is selenium.
According to the invention, the deposition of a lead oxide layer provides compensation for the very sensitive selenium layer within the multilayer photoconductor structure. The proposed passivation layer prevents negative effects of the atmosphere on the photoconductor layers and hence disturbing effects on the edge zones. The passivation layer preferably consists of a synthetic material such as polyethylene, polyxylylene (known as Parylene(copyright)) or polyurethane.
In a preferred embodiment of the invention, the edge zone of the matrix of a sensor of the kind set forth which includes a multilayer photoconductor structure is shielded by a guard electrode window as well as protected by a passivation layer.
Further details and advantages of the invention will become apparent from the dependent Claims and the following description in which the embodiments according to the invention as shown in the Figures are further elucidated. Therein: